1. Field of the Invention
The present invention relates to an enhanced heat exchanger apparatus. The present invention relates also to a heat exchanger formed by several such enhanced heat exchanger apparatuses. It relates further to a method of improving a heat transfer.
A particular application of the invention is the introduction of several enhanced heat exchanger apparatuses inside the radiant coil of a steam cracking furnace.
2. Description of the Prior Art
The three modes of heat transfer are conduction, convection and radiation.
In the classical heat exchangers, the heat is transferred from the hot fluid to the cold fluid through a tube wall essentially by two mechanisms: convection and conduction.
The heat transfer rate is a function of the heat surface, the heat transfer coefficient and the temperature difference between the tube wall and the fluid to be heated (cooled).
At present, technical solutions to improve the heat transfer are the use of finned tubes to increase the heat transfer surface or working in a wall developed turbulent fluid flow regime.
In case of heat exchangers operating at high temperature, for example >400° C., and in particular in the radiant coil of the process furnaces for the steam cracking of the hydrocarbons (where the tube wall temperature may reach a value as high as 1150° C. or even more), additional process needs arise.
In a ethylene plant it is fundamental to operate the steam cracking furnaces at conversion and selectivity as high as possible.
High selectivity means to increase the percentage of the more valuable products such as ethylene, propylene, butadiene at the expense of less valuable products (methane, fuel oil, etc.).
High selectivity is achieved if the residence time is low and the temperature of the process gas is high enough to have a good conversion of the feed.
The above goals are achieved by increasing the heat flux (by consequence, the temperature of the metal reaches a value close to its metallurgical constraint).
A higher temperature of the metal leads to undesired events:
High rates of deposit of coke, creep and carburization.
Also in those particular cases, the technology is oriented towards the improvement of the heat transfer coefficient using tubes with inside fins of various shapes (transverse, longitudinal, or with particular angles).
The inconvenience of the use of extended surfaces is the high cost of manufacturing and the difficulty to apply fins inside the radiant coil of existing ethylene cracking furnaces.
Sometimes internal protrusions in the cracking tube may be the cause of coking due to stagnation of feed gas, which leads to over cracking.
The above technique is focused on improving the heat transfer by the convection mechanism.